I. Field of the Invention
The invention generally relates to mobile telephones and in particular to finite impulses response (FIR) filters for use within cellular telephones employing code division multiple access (CDMA) transmission techniques.
II. Description of the Related Art
FIG. 1 is an illustrative block diagram of the variable rate code division multiple access (CDMA) transmission system as described in the Telecommunications Industry Association's Interim Standard TIA/EIA/IS95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System. Data for transmission by transmission system 10 is provided by a variable rate data source 12. In the exemplary embodiment, the variable rate data source is a variable rate vocoder used for the variable encoding of speech signals as described in detail in U.S. Pat. No. 5,414,796 which is assigned to the assignee of the present invention and is incorporated by reference herein.
In the exemplary embodiment, variable rate transmission system 10 transmits data in frames in accordance with TIA/EIA IS-95-A. Variable rate data source 12 receives digitized samples of input speech and encodes the speech to provide packets of encoded speech as illustrated in FIGS. 3A-3D. The output of variable rate data source 12 is the information bits shown in FIGS. 3A-3D. In the exemplary embodiment, variable rate data source 12 provides variable rate packets of data for transmission at four possible rates 9600 bps, 4800 bps, 2400 bps and 1200 bps, referred to herein as full, half, quarter, and eighth rates. Speech samples encoded at full rate contain 172 information bits, samples encoded at half rate contain 80 information bits, samples encoded at quarter rate contain 40 information bits and samples encoded at eighth rate contain 16 information bits.
Referring again to FIG. 1, in the exemplary embodiment, the variable rate packets are provided to packetizer 13 which in the exemplary embodiment selectively appends cyclic redundancy check (CRC) bits and tail bits. As shown in FIG. 3A, when a frame is encoded by the variable rate data source 12 at full rate, packetizer 13 generates and appends 12 CRC bits and 8 tail bits. Similarly, as shown in FIG. 3B, when a frame is encoded by the variable rate data source 12 at half rate, packetizer 13 generates and appends 8 CRC bits and 8 tail bits. As shown in FIG. 3C, when a frame is encoded by the variable rate data source 12 at quarter rate, packetizer 13 generates and appends 8 tail bits. As shown in FIG. 3D, when a frame is encoded by the variable rate data source 12 at eighth rate, packetizer 13 generates and appends 8 tail bits.
The variable rate packets from packetizer 13 are then provided to encoder 14. Encoder 14 encodes the bits of the variable rate packets for error detection and correction purposes. In the exemplary embodiment, encoder 14 is a rate 1/3 convolutional encoder. The convolutionally encoded symbols are then provided to repetition generator 17.
In the exemplary embodiment, repetition generator 17 receives the packets. For packets of less than full rate, repetition generator 17 generates duplicates of the symbols in the packets to provide packets of a constant data rate. When the variable rate packet is half rate, then repetition generator 17 introduces a factor of two redundancy, i.e. each symbol is repeated twice within the output packet. When the variable rate packet is quarter rate, then repetition generator 17 introduces a factor of four redundancy. When the variable rate packet is eighth rate, then repetition generator 17 introduces a factor of eight redundancy.
In the exemplary embodiment, the encoded symbols are provided to CDMA spreader 16, an implementation of which is described in detail in U.S. Pat. Nos. 5,103,459 and 4,901,307 which are assigned to the assignee of the present invention and are incorporated by reference herein. In the exemplary embodiment, CDMA spreader 16 maps six encoded symbols to a 64 bit Walsh symbol and then spreads the Walsh symbols in accordance with a pseudorandom noise (PN) code.
In the exemplary embodiment, repetition generator 17 provides the redundancy by dividing the data packet into smaller sub-packets referred to as "power control groups". In the exemplary embodiment, each power control group consists of 6 Walsh symbols. The constant rate frame is generated by consecutively repeating each power control group the requisite number of times to fill the frame as described above.
The packets are then provided to a data burst randomizer 18 which removes the redundancy from the packets in accordance with a pseudorandom process as described in copending U.S. patent application Ser. No. 08/291,231, filed Aug. 16, 1994 assigned to the assignee of the present invention and incorporated by reference herein. Data burst randomizer 18 selects one of the power control groups for transmission in accordance with a pseudorandom selection process and gates the other redundant copies of that power control group.
Thus, the output from data burst randomizer 18 consists of sequences of gated values with value 0 bracketing sequences of ungated antipodal data with values of +1 and -1. FIG. 4 illustrates a portion of an exemplary transmission signal having long null portions of value 0 bracketed by antipodal portions of +1's and -1's. Data burst randomizer 18 provides the packet to spreader 16.
The packets are provided by spreader 16 to finite impulse response (FIR) filter 20. The operation of an FIR filter can be described generally by equation 1 below: ##EQU1## In the exemplary embodiment, FIR filter 20 is a 4 times oversampled 48-tap FIR 20 filter illustrated in FIG. 2. As shown in FIG. 2, each sample is delayed by one fourth of the period of the input sequence. Thus, there is a four times redundancy in the data stream.
The filtered signal is then provided to digital to analog-converter 22 and converted to an analog signal. The analog signal is then provided to transmitter 24 which upconverts and amplifies the signal for transmission through antenna 26.
Conventionally, FIR filter 20 would be implemented by means of a digital signal processor or specially designed hardware programmed to perform the numerical calculations of equation 1. For portable cellular telephones, however, the power required to operate the processor or specialized hardware may be unacceptably high. So there is a need for a more efficient means of implementing the FIR filter.